There are generally known so-called functional elements that are formed by stacking compound semiconductor layers on top of a substrate having a hexagonal crystal structure such as a sapphire substrate and that function as various devices such as LEDs (light emitting diodes) and HEMTs (hetero-junction filed-effect transistors).
For example, there are conventionally known semiconductor light emitting elements that exploit light emission resulting from recombination of electrons contained in an n-type compound semiconductor layer with holes contained in a p-type compound semiconductor layer on application of a voltage to a semiconductor element having the n-type and p-type semiconductor layers bonded together with an active layer in between.
As such semiconductor light emitting elements, for example, blue light emitting diode elements are commercially available, and since these blue light emitting diode elements uses a direct transition semiconductor that allows efficient recombination of electrons with holes, they offer extremely high light emission efficiency. Thus, they are used for display on home-use electrical appliances, for indication on traffic lights, for illumination, etc.
For example, white light emitting diodes used for display and illumination as described above are fabricated by combining together a blue light emitting diode element and a phosphor (a fluorescent or phosphorescent substance), such as YAG (yttrium aluminum garnet), having a fluorescence wavelength in a region of yellow light.
Here, blue light emitting diodes use a functional element having a stacked structure of nitride semiconductor layers formed in it. Functional elements having a stacked structure of nitride semiconductor layers formed in them which are in common use as blue light emitting diode elements have a structure in which an n-type GaN layer, an active layer, and a p-type GaN layer are stacked in this order on top of a sapphire substrate. Since the sapphire substrate is electrically non-conducting, etching is performed through the p-type GaN at least into the n-type GaN layer, and an n-type side electrode that makes ohmic contact with the n-type GaN layer is provided on an exposed surface of the n-type GaN layer.
When elements formed by use of a substrate having a hexagonal crystal structure such as a sapphire substrate are split into individual chip structures, since vertical splitting at the center of splitting grooves is difficult, the splitting grooves are often given ample margins.
For example, Patent Document 1 listed below discusses a method of fabricating a semiconductor chip whereby a gallium nitride compound semiconductor chip is fabricated from a wafer (substrate) having gallium nitride compound semiconductors stacked on the principal face of a substrate, and discloses a method etc. of, in expectation of splitting grooves going to be formed obliquely, splitting a wafer at displaced processed positions on the top and bottom thereof.
Patent Document 2 listed below discusses the splitting of a GaAs wafer having a cubic crystal structure by scribing and stealth dicing, and discloses a method of, by controlling the inclination of splitting faces, splitting the wafer into a desired split shape, along with a light emitting element array chip having a characteristic shape.
Patent Document 3 listed below discloses a method of splitting a sapphire substrate believed to be a substrate having a hexagonal crystal structure.
Patent Documents 4 and 5 disclose methods of irradiating a sapphire substrate with pulse laser light to split it.
In the present invention, the term “hexagonal crystal structure” is defined to include a trigonal corundum crystal structure as well.
Many documents mention sapphire as having a trigonal corundum crystal structure, a hexagonal crystal structure, or the like. Non-patent Document 1 listed below contains a passage that reads “although [in the figure the crystal structure is] depicted like a hexagonal crystal structure, this crystal can also be depicted with a rhombohedral crystal structure; the crystal structure is thus described by the latter, which exhibits better symmetry” (a rhombohedral crystal structure refers to a trigonal crystal structure).
Intended in the present invention is a crystal substrate containing corundum (sapphire containing no impurity) or an α-Al2O3 crystal system that is supposed to have the following basic properties: having a crystal axis called c-axis, and further having crystal axes called a1-, a2-, and a3-axes which cross one another at an angle of 120 degrees on a plane perpendicular to c-axis; and describable as a hexagonal crystal structure. Sapphire can be represented by such a crystal axis system, and is mentioned has having a “hexagonal crystal structure” in Patent Document 1 mentioned above. Accordingly, in the present invention, the term “crystal having a hexagonal crystal structure” is defined to include sapphire.
On the other hand, GaN and SiC, which will be mentioned later, generally have a hexagonal crystal structure (they may produce a crystal having a cubic crystal structure through a special fabrication method). These are therefore included in what is referred to as having a hexagonal crystal structure in the present invention.
In the following description, with respect to such crystals, the c-axis direction is identified by [0001], the a1-axis direction is identified by [2-1-10], the a2-axis direction is identified by [-12-10], and the a3-axis direction is identified by [-1-120]. In notations like “−1,” the minus sign preceding a number substitutes for the bar placed over the number in the diagrams.    Patent Document 1: JP-A-2005-191551    Patent Document 2: JP-A-2004-260083    Patent Document 3: JP-A-2005-332892    Patent Document 4: JP-A-2006-245043    Patent Document 5: JP-A-2008-098465    Non-patent Document 1: The article “corundum” in Japanese at Wikipedia, http://ja.wikipedia.org/wiki/, as of Jan. 19, 2010